238 Cultivation of Micro-organisms 



Rather large potatoes should be used, the cylinders being 

 cut transversely, so that a number, each about an inch and a 

 half in length, can be cut from one potato. The skin is re- 

 moved from the cylinders by cutting off the ends, after which 

 each cylinder is cut in two by an oblique incision, so as to 

 leave a broad, flat surface. The half-cylinders are placed 

 each in a test-tube previously sterilized, and are exposed 

 three times, for half an hour each, to the streaming steam of 

 the sterilizer. This steaming cooks the potato and also 

 sterilizes it. Such potato cylinders are apt to deteriorate 

 rapidly, first by turning very dark, second by drying so as to 

 be useless. Abbott has shown that if the cut cylinders be 

 allowed to stand for twelve hours in running water before 

 being dispensed in the tubes, they are not so apt to turn dark. 

 Drying may also be prevented by adding a few drops of clean 

 water to each tube before sterilizing. 

 Some workers insert a bit of glass or a 

 pledget of glass wool into the bottom of 

 the tube so as to support the potato 

 and keep it up out of the water. It is 

 not necessary to have a special small 

 chamber blown in the tube to contain 

 this water, only a small quantity of 

 which need be added. The special 

 reservoir increases the trouble of clean- 

 Fig. 47,-Ravend's mg the tubes. 



potato cutter. If the work to be done with potatoes 



must be accurate, it may be necessary 

 to correct their variable reaction, especially if the acids 

 have not been sufficiently removed by the washing in run- 

 ning water already described. 



To do this the cut cylinders are placed in a measured quan- 

 tity of distilled water and steamed for about an hour. The 

 reaction of the water is then determined by titration and 

 the desired amount of sodium hydroxid added to correct the 

 reaction, after which the potatoes are steamed in the cor- 

 rected solution for about thirty minutes before being placed 

 in the tubes. 



A potato-juice has also been suggested, and is of some 

 value. It is made thus: To 300 c.c. of water 100 grams of 

 grated potato are added, and allowed to stand on ice over 

 night. Of the pulp, 300 c.c. are expressed through a cloth 

 and cooked for an hour on a water-bath. After cooking, the 



